1. Causes of motor axial compression
voltage of three-phase AC motor is usually caused by magnetic circuit
imbalance of the motor, which mainly includes the following situations:
In the production process of the motor, the poor stowing process of the
stator core magnetic steel sheet of the motor leads to the uneven
reluctance of the stator and rotor along the circumference of the core,
and the magnetic flux of the cross-link with the rotating shaft is
generated, so as to generate the induced electromotive force.
In the process of using the motor, due to the use environment and the
motor itself manufacturing (rigidity) problems, resulting in the motor
stator and rotor air gap changes, resulting in the change of reluctance,
uneven flux leakage, and the electromotive force induced with the
(3) Electrostatic induction will also cause the motor to produce shaft voltage.If there is a large number of high-voltage equipment around the motor, the motor shaft may produce an induced voltage.Or
in the case of electrification of various signal devices used with the
motor, if the device is weak insulation and contact with the rotating
shaft, the rotating shaft of the motor will produce shaft voltage.
2. Causes of motor shaft current generation
the motor shaft voltage is very low, only 0.5~2 V, but it will form a
voltage path between the motor shaft, the frame and the housing, because
the impedance of the current loop is very small, so it will produce a
large shaft current P.
normal circumstances, there is a lubricating oil film between the shaft
and the bearing, which can play the role of insulation.For lower shaft voltages, the oil film acts as an insulator and does not generate shaft current.However,
when the shaft voltage increases to a certain value, especially when
the motor starts, because the lubricating oil film in the bearing has
not been established stably, the shaft voltage will break through the
oil film to discharge and form a charged circuit.
the motor bearing oil film breaks down, with the increase of bearing
temperature, the number of bearing breakdown points increases, and the
equivalent resistance of the bearing will decrease. The shaft current
will channel the metal contact points between the bearing and the
rotating shaft. Because the contact point is small and the current
density is large, high temperature will be generated instantly and the
bearing will be partially burned.The
fused bearing alloy flying under the action of grinding pressure is
lost, so small pits are produced on the inner surface of the bearing.Usually
because the shaft hardness and mechanical strength is higher than the
bearing alloy, so the bearing inner ring raceway appears washboard like
3. Treatment measures
(1) Reinforce motor stator housing and foundation
the motor disassembly inspection, it is found that due to the
insufficient strength of the motor body, the poor fastness between the
stator core and the shell, the vibration and noise of the motor appear.As the motor shell is generally welded with thin steel plate, it will vibrate obviously when the motor load is large.The vibration wave makes the motor stator housing tremble, accompanied by a large noise.In
order to solve the vibration and noise problem, the motor housing is
strengthened to change the natural frequency of the housing.In
the process of reinforcement, considering the detachable property of
the original motor stator core and casing, the reinforcement plate is
made into a detachable structure, which is not only conducive to
installing the stator core, but also convenient for replacement.
the same time, the foundation of the motor is reinforced again to
strengthen its rigidity and improve the running stability of the motor.The
above measures can eliminate the adverse effect of induction current
caused by the unbalance of the magnetic resistance caused by the
continuous change of the stator and rotor magnetic circuit.Before modification.
(2) Modification of motor end cover
original motor bearing sleeve and end cover are integrated design, and
the non-transmission end is not insulated. Therefore, if the motor
produces shaft voltage during operation, a path will be formed and shaft
current will damage the bearing.In
view of this situation, the end cover of the non-transmission end was
modified, and the original bearing sleeve was replaced with a new
bearing sleeve wrapped by an insulating epoxy resin layer with good
insulation layer can completely cut off the shaft current loop and
eliminate the influence of the shaft current on the bearing.
(3) Efficiency measures
order to completely eliminate the shaft voltage, a carbon brush can
also be installed between the motor shaft extension end and the end
cover. The carbon brush is connected with the end cover and the rotating
shaft to eliminate the induced potential and bearing potential
difference on the shaft.
4. Protective measures to prevent bearing damage
the voltage of the rotating shaft of the field motor to the ground. If
the voltage of the shaft appears, it will be processed according to the
(1) Consider reducing axial potential.The
grounding carbon brush is installed at the shaft end to ensure its
reliable grounding and reliable contact with the rotating shaft. The
rotating fine potential is kept at zero potential to eliminate the shaft
(2) Cut off the shaft current loop.Insulation isolation is added at the non-transmission end bearing seat and bearing bracket to prevent shaft current generation.
(3) Check the damage degree of the insulation of the motor accessory wire.Use
the maintenance opportunity to check and strengthen the wire or gasket
insulation, to avoid the motor accessory wire insulation damage caused
by shaft current generation.
(4) often check the insulation strength of the bearing seat.For
motors that have taken bearing bearing insulation isolation measures,
the insulation strength of bearing pedestal should be measured regularly
with 500 V mehmmeter, which should not be less than 0.5MQ.If the test value exceeds the standard, the insulation isolation should be processed again until the test is qualified.
(5) Grasping the bearing temperature rise at any time to maintain the purity of lubricating oil insulation medium.Frequently
check the motor bearing temperature and bearing lubricating oil
quality, timely analyze the bearing temperature rise according to the
load and ambient temperature, find out the cause of temperature rise and
deal with it in time.For
the bearing motor, if water is found in the lubricating oil, it must be
filtered, otherwise the insulation strength of the oil film will not
meet the requirements, and it is easy to be broken down by low voltage.